Methods for Manufacturing a Semiconductor Device Having a Non-Ohmic Contact Formed Between a Semiconductor Material and an Electrically Conductive Contact Layer

ABSTRACT

An embodiment of a method of manufacturing a semiconductor device includes providing a semiconductor material that comprises SiC and forming an electrically conductive contact layer on the semiconductor material. A non-ohmic contact is formed between the semiconductor material and the electrically conductive contact layer. The electrically conductive contact layer comprises a metal nitride with a nitrogen content between 10 to 50 atomic %. Additional embodiments of manufacturing a semiconductor device are described.

BACKGROUND

Schottky diodes comprising a metal-semiconductor junction are generally used as rectifying devices. In particular, SiC Schottky diodes are increasingly used in the field of power electronics.

It is an object of the present invention to provide an improved semiconductor device comprising a metal-semiconductor junction. Further, it is an object to provide a method of manufacturing such a semiconductor device.

SUMMARY

According to an embodiment, a semiconductor device comprises a semiconductor material having a bandgap larger than 2 eV and less than 10 eV, and a contact layer in contact with the semi conductor material, the contact layer comprising a metal nitride. A non-ohmic contact is formed between the semiconductor material and the contact layer.

According to a further embodiment, a semiconductor device comprises a semiconductor body including a semiconductor material having a bandgap larger than 2 eV and less than 10 eV, and a contact layer in contact with a first surface of the semiconductor body. The contact layer comprises a metal nitride. The contact layer is electrically connected to a first load terminal, and a non-ohmic contact is formed between the semiconductor body and the contact layer. A second surface of the semiconductor body is electrically connected to a second load terminal, the second surface being opposite to the first surface.

According to a further embodiment, a semiconductor device comprises a semiconductor body including a semiconductor material having a bandgap larger than 2 eV and less than 10 eV, and a contact layer in contact with a first surface of the semiconductor body. The contact layer comprises a metal nitride. The contact layer is electrically connected to a first load terminal. A non-ohmic contact is formed between the semiconductor body and the contact layer. A second surface of the semiconductor body is electrically connected to a second load terminal, the second surface being opposite to the first surface.

Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles. Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.

FIG. 1A illustrates a cross-sectional view of an example of a semiconductor device according to an embodiment.

FIG. 1B shows a cross-sectional view of a semiconductor device according to a further embodiment.

FIG. 1C shows a cross-sectional view of a semiconductor device according to a further embodiment.

FIG. 1D shows a cross-sectional view of a semiconductor device according to a further embodiment.

FIG. 2A illustrates an example of a current-voltage characteristic of an ohmic contact.

FIG. 2B illustrates a cross-sectional view of a rectifying contact.

FIG. 3 illustrates an energy band diagram of a Schottky contact.

FIG. 4 illustrates an example of a reaction chamber which may be used for manufacturing the semiconductor device according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as “top”, “bottom”, “front”, “back”, “leading”, “trailing” etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims.

The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments.

The terms “wafer”, “substrate” or “semiconductor substrate” used in the following description may include any semiconductor-based structure that has a semiconductor surface. Wafer and structure are to be understood to include silicon, silicon-on-insulator (SOI), silicon-oil sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could as well be silicon-germanium, germanium, or gallium arsenide. According to other embodiments, diamond, silicon carbide (SiC) or gallium nitride (GaN) may form the semiconductor substrate material.

The Figures and the description illustrate relative doping concentrations by indicating “−” or “+” next to the doping type “n” or “p”. For example, “n⁻” means a doping concentration which is lower than the doping concentration of an “n”-doping region while an “n⁺”-doping region has a higher doping concentration than an “n”-doping region. Doping regions of the same relative doping concentration do not necessarily have the same absolute doping concentration. For example, two different “n”-doping regions may have the same or different absolute doping concentrations. In the Figures and the description, for the sake of a better comprehension, often the doped portions are designated as being “p” or “n”-doped. As is clearly to be understood, this designation is by no means intended to be limiting. The doping type can be arbitrary as long as the described functionality is achieved. Further, in all embodiments, the doping types can be reversed.

As employed in this specification, the terms “coupled” and/or “electrically coupled” are not meant to mean that the elements must be directly coupled together—intervening elements may be provided between the “coupled” or “electrically coupled” elements. The term “electrically connected” intends to describe a low-resistive electric connection between the elements electrically connected together.

As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

The terms “lateral” and “horizontal” as used in this specification intends to describe an orientation parallel to a first surface of a semiconductor substrate or semiconductor body. This can be for instance the surface of a wafer or a die.

The term “vertical” as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of the semiconductor substrate or semiconductor body.

FIG. 1A shows a cross-sectional view of a semiconductor device 1 according to an embodiment. The semiconductor device 1 illustrated in FIG. 1A comprises a semiconductor material 100 and a contact layer 130 in contact with the semiconductor material 100. As will be explained in the following, the contact layer 130 comprises a metal nitride and a non-ohmic contact is formed between the semiconductor material 100 and the contact layer 130. A bandgap of the semiconductor material 100 is larger than 2 eV and less than 10 eV, e.g. less than 6 eV. According to a further embodiment, the bandgap of the semiconductor material may be larger than 0.9 or 1 eV and less than 10 eV.

The semiconductor material 100 may be a semiconductor body 101 including one or more doped portions or layers at either side thereof. The doped portion may be formed by various methods such as ion implantation, diffusion and epitaxial growth of the doped layer. For example, the semiconductor material may be a material having a bandgap larger than 1 eV. By way of example, the semiconductor material may comprise silicon carbide, diamond, gallium nitride, indium phosphide, AlGaAs and further examples of III-V semiconductors.

For example, the semiconductor body 101 may be heavily n-doped and may comprise a portion that is n-doped at a lower doping level, the portion being disposed at a first surface 110 of the semiconductor body. The contact layer 130 may comprise a combination of a stoichiometric compound with a non-stoichiometric compound including the metal and nitrogen. For example, the contact layer 130 may comprise mixture of M_(x)N_(y) having different values for x and y, wherein M denotes the metal. For example, the metal nitride layer may comprise a mixture of MN and M_(x)N_(y) or of MN₂ and M_(x)N_(y). Generally, in these formulas, x may be equal to 1, and y may be a real number fulfilling 0<y<3. Alternatively, y may be equal to 1, and x may be a real number fulfilling 0<x<3. For example, the metal may be selected from the group of molybdenum, titanium, tantalum, and tungsten. Further, the metal nitride may comprise two metals, such as MoTiN.

The contact layer 130 may be electrically connected to an anode terminal. Further, the semiconductor device 1 may comprise a back side metallization 160 which forms an ohmic contact to the semiconductor body 101. The back side metallization 160 is disposed at a second surface 115 of the semiconductor body 101, opposite to the first surface 110. The back side metallization 160 may be electrically connected to a cathode terminal. The terms “ohmic contact”, “Schottky contact”, and “rectifying contact” will be explained below, while referring to FIGS. 2A and 2B.

FIG. 1B shows a further embodiment of a semiconductor device 1. The semiconductor device 1 illustrated in FIG. 1B comprises a semiconductor material 200, e.g. a semiconductor body 201 and a contact layer 130 in contact with the semiconductor material. The contact layer 130 comprises a metal nitride and a non-ohmic contact is formed between the semiconductor material 200 and the contact layer 130. Generally, the semiconductor material 200 may be a semiconductor body 201 including further doped portions. For example, these doped portions may be disposed adjacent to a first surface 210 or a second surface 215 of the semiconductor body 201. Differing from the embodiment of FIG. 1A, the semiconductor device 1 of FIG. 1B further comprises a doped region 180 of the second conductivity type. For example, the semiconductor material 200 may be n⁻-doped and the doped portion 180 may be p⁺-doped. The doped portion 180 may be disposed at the first surface 210 of the semiconductor body 201, and portions of the semiconductor material 200, which is e.g. n⁻-doped may be present at the first surface 210. The contact layer 130 may be in contact with the semiconductor material 200 and the doped portions 180. The semiconductor device 1 illustrated in FIG. 1B further comprises a heavily n⁺-doped region 170 at the second surface of the semiconductor body 201. The semiconductor device further comprises a back side metallization layer 160 that forms an ohmic contact to the doped layer 170. The back side metallization layer 160 may be electrically connected to a cathode terminal. The contact layer 130 may be electrically connected to an anode terminal. The semiconductor material may comprise any of the materials mentioned above. For example, the semiconductor material may be silicon carbide.

The semiconductor device illustrated in FIG. 1B may implement a Junction-Barrier Schottky (JBS) diode including p⁺ implanted portions 180. When a reverse voltage is applied to the semiconductor device, depletion regions formed at the interface between the n⁻ portion 200 and the p⁺ portion 180 pinch off a leakage current which may arise from the Schottky contact of the device. Accordingly, such a Junction-Barrier Schottky diode has a reduced leakage current. Such a JBS may be suitably used in a switched mode power supply.

According to a further embodiment, the semiconductor device 1 may implement a merged PIN Schottky diode (MPS). FIG. 1C shows a cross-sectional view of such a merged PIN Schottky-Diode. The MPS comprises similar components as the JBS, these components having the same reference numerals as the corresponding components of the JBS. In particular, the p⁺ portions 185 of the MPS are configured to inject minority carriers into the n⁻ portion 200 in a forward direction. For example the p⁺ portions 185 may be doped at a high doping concentration, e.g. 10¹⁹ to 10²⁰ cm⁻³.

FIG. 1D illustrates a further example of a semiconductor device. As is shown, the semiconductor device 1 comprises a semiconductor body 101 comprising a semiconductor material having a band gap larger than 2 eV and less than 10 eV, a contact layer 130 in contact with a first surface 110 of the semiconductor body 101, the contact layer 130 comprising a metal nitride. The contact layer 130 is electrically connected to a first load terminal 240. A non-ohmic contact is formed between the semiconductor body 101 and the contact layer 130. A second surface 115 of the semiconductor body 101 is electrically connected to a second load terminal 250. The second surface 115 is opposite to the first surface 110.

For example, the contact layer 130 may be in contact with the doped portion 120. According to an embodiment, the semiconductor body 101 may be heavily n⁺-doped and may be of an n-conductivity type. The doped portion 120 may be of an n-conductivity type, at a lower concentration of the n-type dopants.

For example, the semiconductor device shown in FIG. 1D may implement a Schottky-diode or a Schottky-diode related device. In this case, the first load terminal 240 may be an anode terminal, and the second load terminal 250 may be a cathode terminal. Depending from different implementations of the semiconductor device, the first load terminal 240 may be a source terminal and the second load terminal 250 may be a drain terminal, e.g. in the case of a MOSFET (metal oxide semiconductor field effect transistor) or a JFET (junction field effect transistor). According to a further example, the first load terminal 240 may be an emitter terminal, and the second load terminal 250 may be a collector terminal, e.g. in the case of an IGBT (insulated gate bipolar transistor).

The semiconductor device may comprise an active region 181 and a junction terminal area 182. In the active region 181, the contact layer 130 is in contact with the semiconductor body 101. The junction terminal area 182 is different from the active region 181 with regard to function and structure. To be more specific, in the active region 181, a load terminal of the semiconductor device, e.g. the anode terminal is electrically connected to the semiconductor body for the purpose of current conduction. In contrast, the purpose of the junction termination area is edge termination for reducing the electric field peak at the periphery of the semiconductor device 1. Typical structural elements of the junction termination area include one or more of field plates, ring structures such as floating guard rings or ring segments, junction termination extension (JTE) structures and variation of lateral doping (VLD) structures, for example.

FIG. 2A shows an example of a current-voltage characteristic of an ohmic contact. As can be seen, the current is approximately proportional with respect to the applied voltage. The ratio of voltage and current is denoted as the resistance of the contact.

On the other hand, as is illustrated in FIG. 2B, across a non-ohmic contact the current need not be proportional with respect to the voltage. Rather, as can be seen on the left-hand side of the chart illustrated in FIG. 2B, almost no current may be flowing, independent from the negative voltage applied. Further, when applying a positive voltage, the current may increase in a non-linear manner. Any kind of current-voltage characteristics in which the current is non-linear to the applied voltage, may be regarded as establishing a non-ohmic contact. For example, the contact may be a rectifying contact such as, for example, a pn junction or a Schottky junction, in which only a small current, i.e. the reverse saturation current flows, when a low voltage in a reverse direction is applied. When a higher voltage is applied in the reverse direction, a breakdown current may flow.

In the context of the present specification, the term “non-ohmic contact” is understood to represent any kind of contact having a non-linear current-voltage characteristics. According to a further modification, the term “rectifying contact” is considered to represent any kind of contact according to which only a little or no current flows, when a voltage in a reverse direction is applied, the current not being proportional with respect to the applied voltage.

FIG. 3 shows an example of an energy band diagram of a rectifying metal semiconductor junction. The right-hand side of FIG. 3 shows the energy band diagram within the semiconductor material, wherein W_(C) denotes the energy level of the conduction band, W_(V) denotes the energy level of the valence band and W_(F) denotes the Fermi level of the semiconductor material. The difference ΔW between the energy level W_(C) of the conduction band and the energy level W_(V) of the valence band denotes the bandgap of the semiconductor material. The left-hand portion of the energy band diagram of FIG. 3 shows the work function q×φ_(M) of the metal. When the metal and the semiconductor material form a junction, a potential barrier is generated at the interface between the Fermi level of the metal W_(f) and the valence band of the semiconductor material. The height of the potential barrier q×φ_(B) also is referred to as the “Schottky barrier” of the contact.

Generally, Schottky contacts including a semiconductor material having a wide bandgap have a large forward voltage drop due to the work function and the Schottky barrier of the contact metals used. According to the described embodiment, by selecting a contact layer including a metal nitride, the height of the Schottky barrier may be adjusted. In particular, by varying the nitrogen content of the metal nitride, the work function of the metal may be suitably set. As a consequence, the Schottky barrier and hence, the forward voltage drop may be set by setting the nitrogen content of the metal nitride. For example, the nitrogen content of the metal nitride may be more than 10 atomic % and less than 50 atomic %, more specifically, from 38 to 45 atomic %. For example, the nitrogen content may be determined using Auger ion spectroscopy, secondary ion mass spectroscopy (SIMS) or X-Ray Photoelectron Spectroscopy (XPS).

The semiconductor device may be a semiconductor component which may be selected from the group consisting of a Schottky diode, a merged pn Schottky diode, a JFET, a MESFET, an integrated flyback diode, a rectifier, an inverter and a power supply.

FIG. 4 illustrates a reaction chamber of a sputter processing apparatus in which the contact layer comprising a metal nitride may be formed. A semiconductor substrate 430 may be disposed on a rotatable table 440. A sputter target 410 may be attached to a supporting element 415. The target may comprise the metal that forms the metal nitride. For example, the target may be made of a metal selected from the group consisting of molybdenum, titanium, tantalum, and tungsten. Further, the target 410 may comprise a combination of these metals. The chamber 400 comprises a gas inlet 420 through which a plasma forming inert gas such as argon may be fed into the reaction chamber. Further, nitrogen (N₂) may be fed via the inlet 420. After igniting a plasma nitrogen is a reactive gas which reacts with the atoms of the target 410. An electrical field as well as a magnetic field may be applied to the sputtering apparatus. Further details of the sputtering method are generally known.

According to an embodiment, by setting the partial pressure of nitrogen, the content of the nitrogen in the deposited metal nitride layer may be determined. It has been shown, that thereby the work function of the contact layer may be changed. For example, the barrier height of a Mo_(x)N_(y) metal in contact with a silicon carbide layer may be 0.94 eV to 1.12 eV. For example, the total pressure within the sputtering chamber may be 4 to 15 mTorr. The partial pressure of nitrogen (N₂/(N₂+Ar)) may be 0.1 to 1.0.

While embodiments of the invention have been described above, it is obvious that further embodiments may be implemented. For example, further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A method of manufacturing a semiconductor device, the method comprising: providing a semiconductor material that comprises SiC; and forming an electrically conductive contact layer on the semiconductor material, wherein a non-ohmic contact is formed between the semiconductor material and the electrically conductive contact layer, wherein the electrically conductive contact layer comprises a metal nitride with a nitrogen content between 10 to 50 atomic %.
 2. The method of claim 1, wherein the electrically conductive contact layer is formed by a reactive sputtering process using nitrogen as a reactant.
 3. The method of claim 2, wherein a partial pressure of nitrogen during the reactive sputtering process is set so as to form the metal nitride as a mixed phase of MN and M₂N, and wherein M denotes the metal.
 4. The method of claim 3, wherein a ratio of the partial pressure of nitrogen and a total pressure during the reactive sputtering process is in a range of 0.1 to 1.0.
 5. The method of claim 3, wherein the partial pressure of nitrogen during the reactive sputtering process is set so as to set a work function of the electrically conductive contact layer.
 6. The method of claim 1, wherein the metal nitride comprises a metal selected from the group consisting of molybdenum, titanium, tantalum and tungsten.
 7. The method of claim 1, wherein the metal nitride comprises two metals.
 8. The method of claim 1, wherein the metal nitride comprises molybdenum.
 9. The method of claim 1, wherein the metal nitride comprises a combination of a stoichiometric compound and a non-stoichiometric compound including a metal and nitrogen.
 10. The method of claim 1, wherein the metal nitride layer comprises a mixture of M_(x)N_(y) and at least one of MN and MN₂, and wherein either x=1 and 0<y<3 or y=1 and 0<x<3.
 11. The method of claim 1, wherein the non-ohmic contact is a rectifying contact.
 12. The method of claim 1, wherein the non-ohmic contact is a Schottky contact.
 13. The method of claim 12, wherein the semiconductor material is a semiconductor body, wherein a doped portion is embedded in the semiconductor body, wherein the doped portion has a conductivity type opposite to a conductivity type of the semiconductor material, wherein the doped portion is in contact with the electrically conductive contact layer and with the semiconductor material, and wherein the doped portion has a conductivity type opposite to the conductivity type of the semiconductor material.
 14. The method of claim 1, wherein the nitrogen content of the electrically conductive contact layer is between 38 to 45 atomic %.
 15. A method of manufacturing a semiconductor device, the method comprising: providing a semiconductor material that has a bandgap in between at least 2 eV and at most 10 eV and comprises SiC; and forming an electrically conductive contact layer that comprises a metal nitride on the semiconductor material, wherein a non-ohmic contact is formed between the semiconductor material and the electrically conductive contact layer, wherein forming the electrically conductive contact layer comprises selecting the composition ratio of the metal nitride.
 16. The method of claim 15, wherein a composition ratio is selected such that a nitrogen content of the metal nitride is between 10 and 50 atomic %.
 17. The method of claim 16, wherein the nitrogen content of the metal nitride is between 38 to 45 atomic %.
 18. The method of claim 15, wherein the metal nitride directly contacts the semiconductor material.
 19. A method of manufacturing a semiconductor device, the method comprising: forming a contact layer in contact with a semiconductor material, the semiconductor material having a bandgap larger than 2 eV and less than 10 eV and comprising SiC, the contact layer comprising a metal nitride, a nitrogen content of the metal nitride being between 38 and 45 atomic %; and forming a non-ohmic contact between the semiconductor material and the contact layer.
 20. The method of claim 19, wherein the metal nitride directly contacts the semiconductor material. 